Electrophotographic copying machines have been making steady advances year by year in copying speed and those which can copy papers of various sizes have been developed. Accordingly, a high-performance electrophotographic photoreceptor which can cope with this has been demanded.
In recent years, with respect to electrophotographic photoreceptors of separate function type in which a plurality of elements individually perform the required functions, various proposals have been made in order to improve the electrophotographic characteristics such as charge retention, stability on repeated use, optical response, spectral characteristics, and mechanical strength.
Various charge generating materials have hitherto been used in the photosensitive layer of an electrophotographic photoreceptor. They include inorganic photoconductive substances such as selenium, zinc oxide, and cadmium sulfide on one hand, and organic photoconductive substances such as organic pigments on the other hand. In particular, the latter are widely used because of the productivity, low cost, and safety while organic electrophotographic photoreceptors in which an organic pigment is used are not always satisfactory in terms of sensitivity, spectral characteristics, and stability on repeated use. In this respect, selenium is especially excellent in terms of sensitivity. In particular, trigonal selenium is excellent in terms of various performance properties required for a charge generating material. That is, trigonal selenium has such advantages that it has strong light absorption over a broad wavelength region to generate a carrier at a high efficiency, it exhibits high chemical stability, and it hardly undergoes deterioration on exposure to heat or light.
Electrophotographic photoreceptors using such trigonal selenium are already known and are disclosed in JP-A-54-54038 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), for example. Also, in JP-A-1-124862 is disclosed an electrophotographic photoreceptor having a photosensitive layer made by dispersing trigonal selenium in the binder resin.
The photosensitive layer of an electrophotographic photoreceptor is required to have the following performance properties. That is, performance properties such as (1) high photosensitivity, (2) low residual potential, and (3) little variation (stability) of photosensitivity, residual potential, and charging potential on repeated use, are required as particularly important. With the photosensitive layer in which trigonal selenium is used as a charge generating material, the characteristics of trigonal selenium play a predominant factor on the above-mentioned requirements (1) to (3). Electrophotographic photoreceptors in which trigonal selenium is used that have been so far proposed, however, do not satisfy all of the requirements (1) to (3), still needing improvements. As an improving means, it has been proposed for example to increase the carrier generation efficiency by giving trigonal selenium a specific crystal structure, and several crystal forms of trigonal selenium having high photosensitivity have been found. Nevertheless, it is the present situation that the technical complicatedness of the conditions for preparing such trigonal selenium and the conditions for preparing the photoreceptor has hindered the development of an electrophotographic photoreceptor which satisfies all the requirements including chargeability, photosensitivity, and stability on repeated use.
The present invention has been made based on the present state of art which has been described above.